1. Field of the Invention
The present invention relates to a semiconductor device manufacturing method and, more particularly, a semiconductor device manufacturing method including a step of growing a local oxidation of silicon (LOCOS) layer on a surface of a semiconductor substrate by using a silicon nitride film as a mask.
2. Description of the Prior Art
A LOCOS layer which is formed on a surface of a semiconductor substrate is employed as a field isolation film for isolating respective devices. A thickness of the field oxide layer is formed differently according to respective portions of device isolation.
When the field oxide layers which have different thicknesses in a plurality regions on the surface of the semiconductor substrate are formed, the thickness of the field oxide layers can be controlled according to a method shown in FIGS. 1A to 1D, for example.
First, as shown in FIG. 1A, a silicon oxide film acting as a pad 2 is formed by thermally oxidizing a surface of a silicon substrate 1. In turn, a silicon nitride layer 3 is formed on the pad 2 by CVD. This silicon nitride layer 3 is formed to prevent oxidation of the surface of the silicon substrate 1. Then, a first window 3a is formed over a first region A, in which a first field oxide layer is to be formed, by patterning the silicon nitride layer 3 by means of photolithography.
Then, as shown in FIG. 1B, a first field oxide layer (LOCOS layer) 4 is formed by oxidizing the surface of the silicon substrate 1 below the first window 3a. This oxidation is the first oxidation step of forming a field oxide layer. For example, this oxidation is performed by heating the surface of the silicon substrate 1 up to 1000xc2x0 C. and supplying oxygen to the surface of the silicon substrate 1 via the first window 3a of the silicon nitride layer 3. In addition, upon supply of the oxygen, chlorine is added at about 0.25 flow rate percents in the course of oxidation. Then, supply of oxygen and chlorine is suspended, then the temperature of the silicon substrate 1 is increased up to 1125xc2x0 C. Then, after the temperature of the silicon substrate 1 has been stable, the silicon substrate 1 is oxidized in an oxygen atmosphere to which chlorine is added at 0.05 flow rate percents, whereby a thickness, of the first field oxide layer 4 is increased.
During such oxidation, a surface of the silicon oxynitride layer 3 is oxidized slightly to thus form a silicon oxynitride film 5.
Then, as shown in FIG. 1C, second windows 3b are formed over second regions B, in which second field oxide layers are to be formed respectively, by patterning the silicon nitride layer 3 by means of second photolithography.
Then, as shown in FIG. 1D, second field oxide layers (LOCOS layer) 6 are grown by oxidizing the surface of the silicon substrate 1 below the second windows 3b and simultaneously the thickness of the first field oxide layer 4 below the first window 3a is increased. This oxidation is a second oxidation step which is performed under the same conditions as the first oxidation step.
As described above, according to patternings of the silicon nitride layer 3 and oxidations of the silicon substrate 1 both carried out twice, two types of filed oxide layers 4, 5 having different thickness can be formed in different regions A, B on the surface of the silicon substrate 1.
However, according to above mentioned steps, as shown in FIG. 2, sometimes a projection 6a is generated on a part of top ends of bird""s beak of the second field oxide layer 6.
For this reason, when a thin silicon oxide film 7 is formed by thermally oxidizing the surface of the silicon substrate 1 again after the silicon nitride layer 3 and the pad 2 have been removed on the substrate 1, a thickness of such thin silicon oxide film 7 is made locally small on the projection 6a formed on the surface of the bird""s beak.
Therefore, if the silicon oxide film 7 is employed as a tunnel insulating film, carriers are apt to leak from such locally thinned area of the silicon oxide film 7 on the projection 6a. 
Such generation of the projection on the bird""s beak of the LOCOS, as mentioned above, has been found by the inventors of the present invention. It may be supposed that such generation is due to stress in the silicon oxynitride layer 5 which is formed on the surface of the silicon nitride layer 3. More particularly, when the window 3b is formed in the silicon nitride layer 3 on which the silicon oxynitride layer 5 has been formed, stress is applied to the silicon nitride layer 3 because of stress of the silicon oxynitride layer 5, and then a crack 3c shown in FIG. 3 is easy to be generated on a side surface of the silicon nitride layer 3 exposed from the window 3b. Therefore, if the surface of the silicon substrate is oxidized via the window 3b later, silicon oxynitride is also generated in the silicon nitride layer 3 along the crack 3c, so that such silicon oxynitride generated in the silicon nitride layer 3 is combined integrally with the bird""s beak of the LOCOS layer 6. As a result, after the silicon nitride layer 3 has been removed, such silicon oxynitride still remains as the projection 6a in vicinity of the top ends of the bird""s beak.
It is an object of the present invention to provide a method of manufacturing a semiconductor device which is able to prevent generation of projection on bird""s beak of a LOCOS.
According to the present invention, in the steps of forming windows at least two times and forming field oxide layers two times, e.g., forming a first window on an oxidation preventing film on a semiconductor substrate; then forming a first field oxide layer by oxidizing a surface of the semiconductor substrate via the first window, then forming second windows on the oxidation preventing film, then forming a second LOCOS layer via the second windows and also increasing a thickness of the first field oxide layer, etc., a chroride acid gas or chlorine gas is contained in a first oxidation gas and a second oxidation gas respectively. At that time, the content of the chlorine gas used in oxidation is contained larger in the second oxidation gas than the first oxidation gas.
When the chlorine is increased in the oxidizing step, an ability of oxidizing the surface of the semiconductor substrate can be enhanced but an ability of oxidizing the oxidation preventing layer can be reduced. Therefore, a growth rate of the field oxide layer via the second window can be enhanced whereas an area along a crack generated in the oxidation preventing layer is difficult to be oxidized. As a result, different oxide is hard to be formed on a surface of bird""s beak of the LOCOS layer, so that generation of the projection formed of such different type of oxide can be prevented.
When the chlorine or chloride gas is set to more than 0.5 flow rate percents relative to a total gas flow rate when an oxygen gas and the chloride gas are supplied to an oxidation atmosphere, an effect of preventing the oxidation of the LOCOS becomes remarkable. In this case, such effect appears more remarkably if the substrate temperature is set to less than 1000xc2x0 C. during oxidation.